Based on https://github.com/objcio/collection-view-swiftui/blob/master/FlowLayoutST/ContentView.swift

CV+ScrollView.swift

//
//  ContentView.swift
//  FlowLayoutST
//
//  Created by Chris Eidhof on 22.08.19.
//  Copyright © 2019 Chris Eidhof. All rights reserved.
//
import SwiftUI

struct FlowLayout {
    let spacing: UIOffset
    let containerSize: CGSize

    init(containerSize: CGSize, spacing: UIOffset = UIOffset(horizontal: 10, vertical: 10)) {
        self.spacing = spacing
        self.containerSize = containerSize
    }

    var currentX = 0 as CGFloat
    var currentY = 0 as CGFloat
    var lineHeight = 0 as CGFloat

    mutating func add(element size: CGSize) -> CGRect {
        if currentX + size.width > containerSize.width {
            currentX = 0
            currentY += lineHeight + spacing.vertical
            lineHeight = 0
        }
        defer {
            lineHeight = max(lineHeight, size.height)
            currentX += size.width + spacing.horizontal
        }
        return CGRect(origin: CGPoint(x: currentX, y: currentY), size: size)
    }

    var size: CGSize {
        return CGSize(width: containerSize.width, height: currentY + lineHeight)
    }
}

func flowLayout<Elements>(for elements: Elements, containerSize: CGSize, sizes: [Elements.Element.ID: CGSize]) -> [Elements.Element.ID: CGSize] where Elements: RandomAccessCollection, Elements.Element: Identifiable {
    var state = FlowLayout(containerSize: containerSize)
    var result: [Elements.Element.ID: CGSize] = [:]
    for element in elements {
        let rect = state.add(element: sizes[element.id] ?? .zero)
        result[element.id] = CGSize(width: rect.origin.x, height: rect.origin.y)
    }
    return result
}


func singleLineLayout<Elements>(for elements: Elements, containerSize: CGSize, sizes: [Elements.Element.ID: CGSize]) -> [Elements.Element.ID: CGSize] where Elements: RandomAccessCollection, Elements.Element: Identifiable {
    var result: [Elements.Element.ID: CGSize] = [:]
    var offset = CGSize.zero
    for element in elements {
        result[element.id] = offset
        let size = sizes[element.id] ?? CGSize.zero
        offset.width += size.width + 10
    }
    return result
}


struct CollectionView<Elements, Content>: View where Elements: RandomAccessCollection, Content: View, Elements.Element: Identifiable {
    var data: Elements
    var layout: (Elements, CGSize, [Elements.Element.ID: CGSize]) -> [Elements.Element.ID: CGSize]
    var content: (Elements.Element) -> Content
    @State private var sizes: [Elements.Element.ID: CGSize] = [:]

    private func bodyHelper(containerSize: CGSize, offsets: [Elements.Element.ID: CGSize]) -> some View {
        ZStack(alignment: .topLeading) {
            ForEach(data) {
                PropagateSize(content: self.content($0), id: $0.id)
                    .offset(offsets[$0.id] ?? CGSize.zero)
                    .animation(.default)
            }
            Color.clear
                .frame(width: containerSize.width, height: containerSize.height)
                .fixedSize()
        }.onPreferenceChange(CollectionViewSizeKey.self) {
            self.sizes = $0

        }.background(Color.red)
    }

    var body: some View {
        GeometryReader { proxy in
            self.bodyHelper(containerSize: proxy.size, offsets: self.layout(self.data, proxy.size, self.sizes))
        }
    }
}

struct CollectionViewSizeKey<ID: Hashable>: PreferenceKey {
    typealias Value = [ID: CGSize]

    static var defaultValue: [ID: CGSize] { [:] }
    static func reduce(value: inout [ID:CGSize], nextValue: () -> [ID:CGSize]) {
        value.merge(nextValue(), uniquingKeysWith: { $1 })
    }
}

struct PropagateSize<V: View, ID: Hashable>: View {
    var content: V
    var id: ID
    var body: some View {
        content.background(GeometryReader { proxy in
            Color.clear.preference(key: CollectionViewSizeKey<ID>.self, value: [self.id: proxy.size])
        })
    }
}

// todo hack
extension String: Identifiable {
    public var id: String { self }
}

struct ContentView: View {
    let strings: [String] = (1...10).map { "Item \($0) " + String(repeating: "x", count: Int.random(in: 0...10)) }
    @State var dividerWidth: CGFloat = 100

    var body: some View {
        VStack {
            HStack {
                Rectangle()
                    .fill(Color.red)
                    .frame(width: dividerWidth)
                ScrollView {
                    CollectionView(data: strings, layout: flowLayout) {
                        Text($0)
                            .padding(10)
                            .background(Color.gray)
                    }
                }
            }
            Slider(value: $dividerWidth, in: 0...500)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Did you manage to understand why it's giving that warning? I'm getting the same warning "Bound preference WidthPreferenceKey tried to update multiple times per frame."

@nachonavarro gives me the same warning did you find any solution?

I still have the warning, but it seems harmless in some cases. In others, I now force a collection change in onAppear and asynchronously undo the change immediately after. It's not nice, but it makes sure the view doesn't break.